Optimisation for data transmission

ABSTRACT

An optimisation method is presented for the transmission of data along any radio frequency link which can be split into distinct transmission blocks, an example being a beam hopping system. By reordering the packets to be transmitted, it is possible to send packets either at, or nearer to, their optimal modulation and encoding configuration. This will allow for a higher bit to symbol conversion for the majority of packets and hence more data bits can be sent for the same number of symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a 35 U.S.C. § U.S. NationalStage Application of International Application No. PCT/EP2021/051443,entitled “OPTIMISATION FOR DATA TRANSMISSION”, filed Jan. 22, 2021,which claims priority to European Application No. 20155446.6, entitled“OPTIMISATION FOR DATA TRANSMISSION”, filed Feb. 4, 2020, the contentsof each being incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to preparation of data for transmission,and particularly, but not exclusively, to radio frequency transmissionof data via a beam hopping scheme.

BACKGROUND OF INVENTION

In many communication systems, several different geographic areas can beserved by a single satellite which transmits data over a downlink toeach geographic area in turn, via a sequence of short transmissionbursts.

For data transmission, an antenna of such a satellite is controlledaccording to a beam-hopping sequence which defines the set of geographiccoverages, representing the areas, or cells, on Earth which are coveredby a particular satellite beam transmitted from the antenna at aparticular time, and a corresponding set of dwell times defining theperiod of time for which the satellite beam should maintain a particulargeographical coverage. Appropriate control of beamforming networks inthe satellite leads to the transmission beam being switched orredirected towards the next cell in the sequence, for a correspondingdwell time. Switching continues in this manner until the end of thepredefined sequence, at which point the sequence is restarted in theabsence of an instruction to the contrary.

In this manner, a number of different ground stations at differentrespective locations may be in communication with the same satellite ona time division basis, ensuring all of the locations can be served bythe available hardware in the satellite communications system. Theswitching is typically rapid, with dwell times of the order of a fewmilliseconds, and the switching can be employed in conjunction withpower or spectrum management in order to share service resources amongdifferent regions efficiently.

In the example described above, the antenna switching sequence may bereferred to as a beam hopping sequence (BHS), in which a satellite beamis “hopped” between coverage areas, or cells, based on a predeterminedsequence of dwell times.

Within a particular cell, requirements of different users may differ. Insome transmission systems, no optimisation is carried out to account forsuch variations. A common modulation and encoding (MODCOD) scheme ofhigh robustness will used for transmissions across all geographicalareas, with the result that the some data will be transmitted at asub-optimal low-efficiency MODCOD.

A MODCOD may be defined by a number of different parameters, such as amodulation type and degree of forward error correction, and is typicallycharacterised in terms of robustness, at the expense of transmissionspectral efficiency.

For ground station terminals experiencing good transmission conditions,such as clear skies, for example, or where the terminal has a largeantenna area, it may be beneficial to use a MODCOD having lowerrobustness since the likelihood of errors caused due to transmissionconditions is reduced, and high robustness may be unnecessary—spectralefficiency can be improved as a result. For terminals experiencing poortransmission conditions, such as heavy cloud cover, or where theterminal has a small antenna area, it may be beneficial to use a morerobust MODCOD using a greater number of symbols to ensure correcttransmission of data.

To solve the problem of use of a sub-optimal MODCOD, it may be possibleto introduce a form of optimisation to ensure that data is transmittedat a MODCOD appropriate for a particular user. Theoretically, this maybe achieved by considering a required MODCOD on a per-packet basis, andtaking appropriate action, but this is unlikely to be feasible inpractical implementations, due to the computational complexity and delayassociated with this process, conflicting with the time available as aresult of the required data transmission rate. For this reason,preferred current solutions are simply to omit optimisation, and toinsert data packets into transmission frames in the order in which theyare arranged at a transmission source, accepting the MODCODinefficiencies.

Embodiments of the present invention aim to address such inefficienciesby transmitting data packets at, or near their optimal MODCOD, in a waywhich is not computationally prohibitive, for ensuring high transmissionrates needed by modern communications systems.

SUMMARY OF INVENTION

According to an aspect of the present invention, there is provided amethod of optimising data for transmission comprising: determining aplurality of data packets to be transmitted at a first transmissionrate; determining required modulation and encoding schemes, MODCODs, forbits of the plurality of data packets; assigning bits to a respectivebaseband transmission frame, BBFRAME, of a sequence of a plurality ofBBFRAMEs, in accordance with the required MODCODs for the respectivebits, such that: (i) each BBFRAME is associated with the highest commonMODCOD robustness of the MODCODs required for each of the bits in theBBFRAME; and (ii) the robustness of the MODCOD associated with eachBBFRAME increases or decreases in accordance with the chronologicalposition in the sequence of the BBFRAMEs; and determining a sequence ofdata packets of the plurality of data packets to be arranged within theBBFRAME sequence in accordance with the assignment of bits to BBFRAMEs,and outputting the sequence of data packets to a buffer for arrangementas a transport stream comprising the sequence of BBFRAMEs

The data transmission may be arranged to occur during a dwell time of abeam hopping schedule of a communications system using a predeterminednumber of symbols.

The method may further comprise determining whether a BBFRAME hascapacity to accommodate bits of the plurality of data packets; if theBBFRAME has capacity, assigning the bits to the BBFRAME; and if theBBFRAME does not have capacity and the dwell time is not filled,assigning the bits to an adjacent BBFRAME in the sequence of BBFRAMEs.

The method may further comprise determining that it is not possible toaccommodate one or more additional bits of the plurality of data packetsin the sequence of BBFRAMEs; determining that optimisation of thesequence of BBFRAMEs is possible if an optimisation limit is notexceeded, the optimisation comprising: identifying further data packetsto be transmitted during the dwell time at a second transmission ratehigher than the first transmission rate; adding further BBFRAMEs to thesequence of BBFRAMEs within the dwell time; and assigning bits of thedata packets and the further data packets to the sequence of BBFRAMEsand further BBFRAMEs, in accordance with the required MODCODs for thebits.

The method may further comprise repeating the optimisation until theoptimisation limit is reached.

The optimisation limit may be the bit-to-symbol conversion rate limitfor the dwell time.

The optimisation limit may be a predetermined number of optimizations,if the bit-to-symbol conversion rate limit for the dwell time is notexceeded.

The method may further comprise: identifying the MODCOD of each BBFRAME;preparing a transmission stream based on the sequence of data packets tobe arranged in the sequence of BBFRAMEs and the MODCOD of each BBFRAME;and transmitting the transmission stream.

The data packets in a BBFRAME may be arbitrary.

The required MODCOD of each data packet may be contained within the datapacket.

According to another aspect of the present invention, there is providedan apparatus comprising one or more processors, and memory storingcomputer-executable instructions that, when executed by the one or moreprocessors, causes the method described above to be performed.

According to another aspect of the present invention, there is provideda satellite payload comprising the above apparatus.

According to another aspect of the present invention, there is provideda computer program which, when executed by a processor, is arranged tocause the method described above to be performed.

By reordering packets in the manner described above, it is possible tosend packets either at, or near to, their optimal MODCOD. This willallow for a higher bit-to-symbol conversion rate for the majority ofpackets, and hence more data bits can be sent for the same number ofsymbols, increasing system throughput.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will be described, by way ofexample only, in connection with the following drawings, of which:

FIG. 1 illustrates a beam hopping system in which optimisation isperformed according to embodiments of the present invention;

FIG. 2 illustrates an optimisation system according to embodiments ofthe present invention;

FIG. 3 illustrates an optimisation method according to embodiments ofthe present invention; and

FIG. 4 illustrates an example of a data stream optimised according toembodiments of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are described in relation to thepreparation of a data stream for transmission over a radio frequency(RF) downlink from a satellite 1 to a plurality of ground stations 2 a,3 a, 4 a. The data stream may represent content, such as audio, video,image information, or other consumable data, and in the embodimentillustrated in FIG. 1 , the data is provided to a plurality of userterminals as ground stations in each of a plurality of different cells2, 3, 4 on a time division basis using a beam hopping scheme.

In embodiments of the present invention, the preparation of atransmission stream involves the ordering of frames of data fortransmission. The data in the ordered frames modulates a predeterminedcarrier signal used in the satellite downlink using a set of symbolsassociated with the modulation system required by the transmission data.

The ordering process involves an optimisation method, described in moredetail below, which is performed on baseband data, prior to itsfrequency upconversion during the preparation of a transport stream, inorder to ensure efficient transmission.

In embodiments of the present invention, data for transmission isarranged in a sequence of data fields referred to herein as basebandframes (BBFRAMEs). Each BBFRAME comprises a plurality of data bits, andthe sequence of BBFRAMEs fills a dwell time in the beam hopping scheme.

Data packets, comprising a plurality of bits, are requested from abuffer. The sequence of BBFRAMEs is filled, in chronological order, withbits from the buffer having similar MODCODs, instead of the order beingrandom. Once the dwell time has been filled, optimisation methods ofembodiments of the present invention will determine whether it ispossible to add more bits to the dwell time as a result of an increasedbit-to-symbol conversion rate deriving from the MODCOD grouping. Theoptimisation method is capable of optimising over as many BBFRAMEs asare required.

The result of the optimisation is therefore a mapping between bits andBBFRAMEs. enabling data packets to be selected and arranged in theBBFRAME sequence. The arrangement of data packets is used to form thetransmission frames.

The optimisation method, described below, is performed on a satellitepayload. FIG. 2 illustrates the architecture of an optimisation system10 according embodiments of the present invention.

The system comprises a processor 12, which may be a central processingunit or may include a set of distributed processors, which executes acomputer program comprising computer-readable instructions in order toperform the method of embodiments of the present invention. Thecomputer-readable instructions to be executed are stored in a memory 14,such as a non-volatile memory (e.g. a read-only memory (ROM)) or arandom access memory (RAM) to be accessed by the processor. Theprocessor may be implemented as an application-specific integratedcircuit (ASIC).

The optimisation system 10 comprises an input buffer 16. The inputbuffer may 16 be any suitable memory, such as a volatile memory, anddata is written into and read from the input buffer 16 under the controlof the processor 12.

External to the optimisation system 10 is data storage 18 which storesdata packets to be transmitted. The data storage 18 may comprise one ormore content servers, for example, which store content to be provided toa number of user terminals. The processor 12 operates to retrieve datapackets from the external data storage 18 and to store the retrievedpackets in the input buffer 16. In alternative embodiments, the datastorage may be contained within the optimisation system 10.

The input buffer 16 stores the total number of bits which are to beinserted into a sequence of BBFRAMEs. When data packets are firstretrieved from the external storage 18, there is no assignment of datato the BBFRAMEs, and as described below in more detail, aninitialisation state is adopted in the sequence of BBFRAMEs is filledwith bits based on MODCOD order.

The dwell time of the beam hopping scheme is predetermined and isprovided to the processor 12 by an external network controller 20 whichcontrols a satellite for transmission. The processor 12 is thereforeable to determine the number of physical layer frames (PLFRAMEs) whichwill fit within the dwell time, for a given transmission rate and forgiven BBFRAME MODCODs. Based on this, the number of bits which can beaccommodated within a BBFRAME can be determined by the processor 12.

As described in more detail below, the processor 12 executes anoptimisation method in order to refine the BBFRAME sequence from theinitialisation state by adding further bits to be transmitted, and tomake further refinements in an iterative process. The iterations aresuch that the transmission rate, the number of BBFRAMEs in the dwelltime, and the corresponding number of data packets to be transmitted,can be increased without violating the required modulation and encodingscheme of the data packets in the transmission.

Once the optimisation method is complete, the processor 12 controls theprovision of the bits and their respectively assigned BBFRAMEs to amodulator 22. With the knowledge of how many bits at each MODCOD shouldbe assigned to each BBFRAME, as output by the optimisation process, themodulator 22 prepares a transport stream by assigning data packets basedon how many bits they contain, and the BBFRAME they fall into(identified via a BBFRAME ID or checksum, for example) given theirrequired MODCOD, and by modulating a predetermined carrier signal. Themodulator 22 is shown outside of the optimisation system 10 of FIG. 2 ,but in alternative embodiments may be inside the optimisation system 10.

Conventional transmission MODCODs, and their robustness, are to be foundin section 6 of the European Telecommunications Standards Institute(ETSI) standard EN 302 307-1 and 307-2, and would be well understood tothose skilled in the art. Examples are phase shift keying (PSK)modulation schemes employing a variety of symbol constellations. Theaforementioned ETSI standard also details the process of physical layerframing for transmission, prior to frequency modulation, with physicallayer frames (PLFRAMES) being constructed from BBFRAMEs to which errorcorrection (e.g. forward error correction) is applied. As such, thetransmission of data packets in a particular sequence of BBFRAMEs isconsidered to be well known in the art, and the transmission stages arenot essential to embodiments of the present invention.

The transmission is executed by providing the transport stream to atransmission interface for a satellite, comprising a plurality ofbeamforming networks which control transmission of data in the dwell,before executing switching to the next cell coverage area in the nextdwell.

The processor 12 may, in some embodiments, be implemented in a satellitepayload, interfacing with an on-board controller of the satellitepayload. In such embodiments, the data packets to be transmitted may bereceived from a ground-based storage, either via the uplink of thesatellite payload, or via one or more other satellites in aconstellation via inter-satellite links.

The processor 12 may, in other embodiments, be implemented in a groundstation, accessing terrestrial external data storages to provide contentfor transmission. In such embodiments, the data packets, once orderedoptimally in the BBFRAME sequence, are provided to a satellite via anuplink for distribution.

The optimisation method performed by the processor 12 may be performedduring a time slot prior to commencement of the servicing of aparticular cell in the beam hopping scheme. Such a time slot may occurwhile the satellite is involved in servicing a different cell within thebeam hopping scheme.

The beam hopping scheme which is executed by the satellite may change inaccordance with scheduled or dynamically-determined changes to accountfor environmental variations, for example. When the scheme changes, thedwell time may change, which may require further iterations to beperformed so to reflect a modified dwell time.

FIG. 3 illustrates an optimisation method according to embodiments ofthe present invention. The method is implemented as an algorithmexecuted by the processor 12 shown in FIG. 2 .

In step S301, data packets are requested from external data storage 18containing data for transmission and stored in the input buffer 16. Abit histogram is constructed from the requested data packets, whichrepresents the number of bits associated with a particular MODCOD(expressed in order of, for example, a robustness parameter).

In step S302 bits of the data packets are added to a sequence ofBBFRAMES in a filling process, by taking bits associated with aparticular bar from the histogram and testing whether the number of bitscan be added to the total number of bits in the BBFRAME. By selectingbits from a particular histogram bar, the bits to be added to aparticular BBFRAME will have a similar MODCOD, as associated with thehistogram bar.

The MODCOD required by a particular packet may be contained within thedata packet itself, such as in the packet header, defined by theterminal for which the data packet is destined. In alternativeembodiments, the required MODCOD may be provided to the optimisationsystem as a setting (e.g. a setting for a particular ground stationterminal), to be accessed via a lookup table in correlation with theidentity of the terminal scheduled to receive the data packet.

In some embodiments, the grouping of bits of similar MODCOD is such thatthe MODCOD of each BBFRAME increases in robustness, relative to aprevious BBFRAME in the sequence. In other embodiments, the grouping ofbits of similar MODCOD is such that the MODCOD of each BBFRAME decreasesin robustness, relative to a previous BBFRAME in the sequence.Transmission requirements determine whether increasing MODCOD ordecreasing MODCOD is required for the sequence of BBFRAMEs.

The filling process continues by adding bits to the next BBFRAME once aparticular BBFRAME is full. In some embodiments, each BBFRAME is filledcompletely as bits from a particular data packet can span a BBFRAMEboundary, in a process referred to herein as “BBFRAME spanning”. Inalternative embodiments, BBFRAME spanning is not used.

Within step S302, a series of tests is performed as part of the fillingprocess, as described below. At step S303, it is determined whether itis possible to add bits to the total number of bits in the last BBFRAMEin the sequence, based on the capacity of the BBFRAME and the number ofbits already filling it. If the BBFRAME has capacity to accommodatefurther bits (S303Y), the bits are added to the total number of bits inthat BBFRAME in step S304, and the process returns to step S303 where itis repeated in relation to the further bits from the input buffer.

If it is determined in S303 that the BBFRAME does not have capacity toaccommodate the bits (S303N), the method proceeds to step S305, where itis determined whether an optimisation limit has been reached, beyondwhich an optimisation function will not be able to produce a moreoptimal solution than the previous optimisation has found, and/or atwhich a maximum processing time for the optimisation process, specifiedas a system requirement, has been reached. At the optimisation limit,ends and the most optimal solution is chosen as the order in which thedata is to be transmitted.

The optimisation limit may, in some embodiments, represent either apredetermined number of iterations (for example, four) of theoptimisation, or may, in other embodiments, represent the bit-to-symbolconversion rate limit. Where bit-to-symbol conversion is used as thebasis of the determination in step S305, it determined whether thenumber of symbols associated with the dwell time of the transmissionsystem is exceeded by the addition of one or more BBFRAMEs.

If further optimisation is possible (S305N), the method continues tostep S306, at which a new BBFRAME is added to the sequence of BBFRAMEs,and the process returns to step S301 where new bits are added to the newBBFRAME sequence.

If no further optimisation (S305Y) is possible, it is determined thatthe dwell time of the transmission has been filled by bits from theinput buffer, and the process proceeds to S307, where the mapping ofbits to BBFRAMEs is used to determine the sequence of data packets fromthe input buffer 16 to be arranged within the BBFRAMEs. The packetsequence is sent to the modulator.

As a result of the MODCOD-based ordering of bits, the bit-to-symbolconversion rate for transmission of the bits will, on average haveincreased in comparison to an unordered sequence. This occurs becausethe sequence of BBFRAMEs is such that there are more BBFRAMEs which aretransmitted at a MODCOD which is more efficient/less robust than wouldbe the case if there had been no MODCOD grouping based on the bithistogram, and the increased efficiency of such BBFRAMEs is associatedwith a reduced proportion of the dwell time.

In comparison to a sequence of BBFRAMEs containing bits in which theMODCOD robustness varies in an arbitrary manner across the entireBBFRAME sequence, the MODCOD of each BBFRAME might be associated with arelatively high robustness due to the fact that each BBFRAME takes thehighest common robustness MODCOD of the bits within it. Consequently,transmission spectral efficiency for BBFRAMEs which are not in MODCODorder is relatively low, in comparison with that achieved by theoptimisation method of embodiments of the present invention, due to thehigher robustness required, and the need for more symbols for aparticular number of bits.

Consequently, the number of symbols any particular transmission needs touse, as a result of embodiments of the present invention, will decrease.The increased efficiency means that the required transmission time isnow smaller, and thus, it is possible to fit more transmission packetsinto the available dwell time, using the same number of symbols. Theavailable transmission rate is therefore increased, dependent on thedetermined bit-to-symbol conversion rate of the optimised BBFRAMEsequence. More bits can be requested from the input buffer fortransmission, and the process returns to step S301.

In step S305Y, information is sent to the modulator representing thefinal arrangement of BBFRAMEs in terms of the number of bits in eachBBFRAME, and the MODCOD associated with each BBFRAME.

In step S307, data packets are selected from the input buffer 16, andassigned to a BBFRAME of the transmission as determined in the finaloptimisation step S306 of the method. The selection of packets is suchthat each BBFRAME of the sequence of BBFRAMEs can be considered to beassociated with a MODCOD, which is the highest common robustness, orlowest common transmission spectral efficiency, of the MODCODs requiredby each of the data packets assigned to that BBFRAME. Since data packetsof similar MODCOD are grouped together, it may be the case, in someembodiments, that the overall MODCOD of the BBFRAME to which groups ofdata packets are assigned does not vary significantly from any of thedata packets within the BBFRAME.

The assignment of data packets to BBFRAMEs is sent to the modulatorwhere the transmission stream is prepared. In the event that a sequenceof BBFRAMEs does not fully fill the dwell time, but in which it is notpossible to add a further BBFRAME, dummy symbols may be added to thetransmission to fill the dwell time.

In Step 308, the modulator 22 prepares a transmission stream as symbolsof a carrier wave having preselected transmission characteristics (e.g.frequency).

If BBFRAME spanning is not used, the BBFRAMEs do not need to betransmitted in the order of the sequence determined by the optimisationmethod, as the BBFRAME ordering is for the purpose of efficient groupingof the MODCODs of the bits, as described above. Once data packets areassigned to a particular BBFRAME, the BBFRAMEs can be transmitted in anyorder, since the improvement of the improved grouping of the datapackets applies irrespective of the transmission order of the groupsthemselves. It will be appreciated that this does not apply if BBFRAMEspanning is used, as transmitted data packets must arrive in order to beassembled. In the case where BBFRAME spanning is not used, when packetsare assigned to BBFRAMEs, dummy bits may also need to be added to eachBBFRAME in cases where a data packet does not exactly fill the wholeBBFRAME.

The transmission step itself need not be part of embodiments of theinvention and can be performed by external systems receiving anassignment of bits to BBFRAMEs, but other embodiments may include thetransmission stage.

The assignment of bits to BBFRAMEs avoids the need to physically movedata packets in the buffer into a particular order as part of theoptimisation process itself. This would otherwise be computationallyexpensive. Consequently, it is possible to iterate the optimisationprocedure by updating only the number of bits at each MODCOD, and onlyonce the final mapping is known are BBFRAMEs assembled. The allocationof bits on the histogram to the appropriate BBFRAME removes the need tosort packets unnecessarily.

An advantage of the method described is that the data packets do notneed to be arranged individually in MODCOD order. The BBFRAMEs are sentin MODCOD order, but within a given BBFRAME, the data packets may bearranged in any order, including an arbitrary order. This flexibilitygreatly reduces the computational time of the method, as nocomputational resources are required to order the packets within theBBFRAMEs.

Techniques according to embodiments of the present invention can beshown to increase transmission efficiency, by over 50% in some cases, interms of the quantity of data transmitted in a fixed time, in comparisonto techniques as described in the introduction for the application, inwhich no packet re-ordering is applied.

Although embodiments of the present invention are described inconnection with a beam hopping system, this is simply an example, andthe optimisation method described above can be applied to transmissionof data along any RF link which can be split into distinct transmissionblocks.

FIG. 4 illustrates an example of the structure of a data stream,achieved using the method described in FIG. 3 . In the embodiments ofFIG. 4 , the data stream to be transmitted may be a logical data streamconforming to the Digital Video Broadcast—Second Generation Terrestrialstandard (DBV-T2). As described above, the data stream which isultimately transmitted is divided into a series of PLFRAMEs, formed fromFECFRAMEs which are in turn formed from BBFRAMEs to which forward errorcorrection check bits are added, using e.g. a low-density parity-check(LDPC) or Bose-Chaudhuri-Hocquenghem (BCH) code, as known in the art.

For a given transmission rate, the FECFRAMEs are of a predetermined bitlength, determined by system requirements. The length of a PLFRAME isdependent on a particular MODCOD associated with the data packets withinthe BBFRAME of the FECFRAME which forms the PLFRAME, and is dependent onthe ratio of bits in the FECFRAME (based on the number of data bits andthe required encoding bits) to symbols. Consequently, as the method ofFIG. 3 proceeds to perform optimisation, the length of a PLFRAME can beshortened by increasing the number of data packets to be transmittedwith a lower-robustness and higher-efficiency MODCOD, such that a higherratio of symbols to FEC bits can be used in comparison with an initialconfiguration state prior to optimisation. As a result of the shorteningof the PLFRAMEs, it becomes possible to fit more PLFRAMEs within thedwell time, which in turn enables more data packets to be assignedrespectively to more BBFRAMEs.

Consider an example in which four FECFRAMEs 41, 42, 43, 44 can beconstructed using bits filling four BBFRAMEs 51, 52, 53, 54, as shown inFIG. 4(a). In this example, BBFRAME spanning is used and the BBFRAMEsare full. Each BBFRAME is associated with the highest robustness of theMODCODs required by the bits within the BBFRAME. Since the FECFRAMElength is fixed, and since the error correction data to be added to theBBFRAME is dependent on the MODCOD associated with the BBFRAME, themaximum number of bits which the BBFRAME can contain is limited independence upon the MODCOD of the BBFRAME. A stream of PLFRAMEs 61, 62,63, 64 is constructed, with dummy symbols 65 added to fill the dwelltime.

The length of PLFRAMEs 61, 62, 63, 64 are not shown to scale. Inpractice, a PLFRAME will not be longer than the corresponding FECFRAMEas the most robust MODCOD has a bit to symbol conversion ratio of 1:1.For the case where this ratio is 1:1, the PLFRAME is the same length asthe FECFRAME. It can be seen that PLFRAME 64, which contains bitsassociated with BBFRAME 54, is of the shortest duration as it isassociated with the smallest amount of encoding, and PLFRAME 63 is ofthe longest duration as it is associated with a higher order modulationscheme.

After optimisation based on one iteration in the method of FIG. 3 , thelength of the PLFRAMEs is shortened as it is possible to use a highertransmission rate. The higher transmission rate is made possible becauseof the higher bit to symbol rate, as described above. As the length ofthe PLFRAMEs are shortened, it is possible to accommodate more PLFRAMEsin the same dwell time.

FIG. 4(b) shows an example in which six BBFRAMEs 71, 72, 73, 74, 75, 76can now be included in the transmission stream, via six PLFRAMEs 81, 82,83, 84, 85, 86. Again the PLFRAMEs are not shown to scale, but theirlength is dependent on the bit to symbol conversion of the FECFRAMElength. The proportion of the PLFRAME taken up by error code is reduced.Again, dummy symbols 87 are used to fill the dwell time.

It will be appreciated that the present invention is not limited to theembodiments described above, and modifications will be apparent whichfall within the scope of the invention defined by the following claims.

The invention claimed is:
 1. A method of optimizing data fortransmission, the method comprising: determining a plurality of datapackets to be transmitted at a first transmission rate; determiningrequired modulation and encoding schemes (MODCODs) for bits of theplurality of data packets; assigning the bits of the plurality of datapackets to a respective baseband transmission frame (BBFRAME) of asequence of BBFRAMEs, in accordance with the required MODCODs forrespective bits, such that each of the BBFRAMEs is associated with ahighest common MODCOD robustness of the required MODCODs for each of thebits in the BBFRAME, wherein a robustness of a MODCOD associated witheach of the BBFRAMEs increases or decreases in accordance with achronological position in the sequence of BBFRAMEs, wherein theassigning of the bits of the plurality of data packets comprises:determining whether a BBFRAME has capacity to accommodate the bits ofthe plurality of data packets; when the BBFRAME has capacity, assigningthe bits to the BBFRAME; and when the BBFRAME does not have capacity anda dwell time is not filled, assigning the bits to an adjacent BBFRAME inthe sequence of BBFRAMEs; and determining a sequence of data packets ofthe plurality of data packets to be arranged within the sequence ofBBFRAMEs in accordance with the assigning of the bits to the BBFRAMEs,and outputting the sequence of data packets to a buffer for arrangementas a transport stream comprising the sequence of BBFRAMEs, wherein theassigning of the bits to the BBFRAMES optimizes the sequence of datapackets for transmission during the dwell time of a beam hoppingschedule of a communications system using a predetermined number ofsymbols, and wherein a required MODCOD is contained within each datapacket of the plurality of data packets.
 2. The method according toclaim 1, further comprising: determining that it is not possible toaccommodate one or more additional bits of the plurality of data packetsin the sequence of BBFRAMEs; determining that optimization of thesequence of BBFRAMEs is possible if an optimization limit is notexceeded, the optimization comprising: identifying further data packetsto be transmitted during the dwell time at a second transmission ratehigher than the first transmission rate; adding further BBFRAMEs to thesequence of BBFRAMEs within the dwell time; and assigning bits of theplurality of data packets and the further data packets to the sequenceof BBFRAMEs and further BBFRAMEs, in accordance with the requiredMODCODs for the bits.
 3. The method according to claim 2 comprisingrepeating the optimization until the optimization limit is reached. 4.The method according to claim 3, wherein the optimization limit is thebit-to-symbol conversion rate limit for the dwell time.
 5. The methodaccording to claim 3, wherein the optimization limit is a predeterminednumber of optimizations, if the bit-to-symbol conversion rate limit forthe dwell time is not exceeded.
 6. The method according to claim 1comprising: identifying the MODCOD of each BBFRAME; preparing atransmission stream based on the sequence of data packets to be arrangedin the sequence of BBFRAMEs and the MODCOD of each BBFRAME; andtransmitting the transmission stream.
 7. The method according to claim6, in which the order of the data packets in a BBFRAME is arbitrary. 8.An apparatus comprising one or more processors, and memory storingcomputer-executable instructions that, when executed by the one or moreprocessors, causes the method of claim 1 to be performed.
 9. A satellitepayload comprising the apparatus of claim
 8. 10. A non-transitorycomputer readable medium including a computer program which, whenexecuted by a processor, is arranged to cause the method of claim 1 tobe performed.
 11. The method according to claim 1, wherein bits from aparticular data packet span a boundary of a respective BBFRAME.
 12. Themethod according to claim 1, further comprising: determining whether anoptimization limit has been exceeded, wherein it is presumed that thedwell time is not filled when the optimization limit has not beenexceeded.
 13. The method according to claim 12, wherein the optimizationlimit has been exceeded when a predetermined number of optimizationiterations have been performed.
 14. The method according to claim 12,wherein the optimization limit has been exceeded when the bit-to-symbolconversion rate limit has been exceeded.
 15. The method according toclaim 12, further comprising: when the optimization limit has beenexceeded, determining a sequence of data packets from an input buffer tobe arranged within the BBFRAMEs.